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In the Tremie Concrete pouring method, Concrete is placed below water or ground level though a pipe, the lower end of which is kept immersed in fresh concrete so that the rising concrete from the bottom displaces the water or Bentonite without washing out the cement content. Tremie can also be used as a noun to refer to the hopper and drop pipes used to place the concrete underwater or below ground level.

Concrete Placement

Three issues are critical to the placement of concrete under water or slurry:

• A delivery method must be used that keeps the concrete separated from the water or slurry and prevents intermixing.

• The concrete must maintain a high slump to completely fill the shaft and flow easily around the reinforcing steel.

• The top of the shaft must be thoroughly flushed at completion of the pour.

Suspension jigs that hold the tremie string in place while the next section is coupled

Various designs of hopper for the opening of the tremie string; style is denoted by the method of suspension or pour

Delivery

Concrete placed underwater or slurry must be placed through a closed, sealed tremie or with a pump. A tremie is a smooth steel pipe, usually 10 to 12 in diameter, with a hopper on top into which concrete is deposited. Concrete is discharged out the bottom of the tremie pipe. At the start of the pour, some means must separate the concrete from the water or slurry. Most commonly, the open tremie is inserted to the bottom of the excavation and allowed to fill with water or slurry. A foam rubber plug, usually referred to as a “pig,” is placed in the top of the tremie pipe before the hopper is filled with concrete. As concrete enters the tremie pipe, the pig slides down, separating the concrete from the water or slurry. Once the tremie pipe and hopper is full of concrete, lifting the tremie slightly allows the pig to be expelled, and concrete begins flowing into the shaft. Concrete with the correct slump will often flow up the tremie a considerable distance (10 to 20 feet) while the hopper is recharged as needed. Once flow slows or stops, the tremie is lifted to restart the flow. It is critically important that the tremie remains well embedded in the concrete. Five feet of embedment should be maintained as a minimum at all times. The inspector should check this using a weighted tape to determine the level of concrete within the shaft and then calculating tremie embedment based on the measured length of the tremie.

Failure to maintain tremie embedment will trap the soil cuttings sediment, and washed-out concrete in the shaft. In a correct pour, these materials remain on top of the concrete and are flushed off at the top of the shaft at the end of the pour. Pumped concrete is placed in a similar manner. The concrete and water must be prevented from mixing at the start of the pour, and a foam rubber plug is often used in the pump line as a separator. Additionally, embedment of the pump line into the concrete is critical throughout the pour.